Variations in the operational parameters of a printer while printing can cause variations in color and tone reproduction between and within jobs. Printers are calibrated by printing test targets of known colors, measuring the colors reproduced by the printer, and comparing those measurements to the known colors to determine correction factors. Flatbed scanners can be used to scan printed targets. However, flatbed scanners are most often colorimetric instruments, not spectroradiometric instruments. That is, flatbed scanners represent each color as a match of three corresponding primaries. Spectroradiometers, by contrast, measure the full spectrum of light reflected from each patch, permitting much more accurate measurements and effective compensations. However, spectroradiometers are most often capable of measuring only one sample at a time; typical test targets include many samples. Spectroradiometer measurements are often taken by an operator, or robotically, one patch at a time, using an instrument such as an I1BASIC from X-RITE, INC. As used herein, a “spot scanner” is a scanner capable of measuring only one patch at a time. Spot scanners can be mounted on mechanical or robotic carriers to scan, e.g., full-page targets, but they still scan only one patch at any given instant of time.
U.S. Pat. No. 6,441,923 to Balasubramanian et al. describes generating a calibration target dynamically in response to selected printer variables. The whole target is measured and used for calibration. This scheme is, therefore, not optimized for use with spot scanners. It requires scanning many patches.
Commonly-assigned U.S. Pat. No. 7,271,935, issued Sep. 18, 2007 to Coons et al., describes a printer for printing color patches of a selected color patch set adjacent to printed images on a receiver medium. A patch set is selected based on the amount of space left on the receiver, measured in terms of the number of gaps between images remaining on the receiver medium. U.S. Pat. No. 7,123,384, issued Oct. 17, 2006 to Koifman, describes a dot-gain calibration target including a plurality of sets of patches, each set screened differently. For example, the different sets can have different pitches in lines per inch (Ipi). Multiple strips are printed on a single target and scanned together. Although useful, these schemes do not assist the operator of a spot scanner.
U.S. Pat. No. 7,069,164, issued Jun. 27, 2007 to Viturro et al., describes calibrating inline sensors in a printer using a reference target containing rows and columns of patches. U.S. Pat. No. 6,567,170, issued May 20, 2003 to Tandon et al., describes an inline sensor for detecting color variations in printed test patches. The sensor includes a color digital camera to image the printed test patches, and one or more LEDs for illumination. Since this scheme uses R, G, B or other spectral bandpass filters, it cannot provide spectral information at the level of accuracy of a spot scanner. Moreover, neither of these schemes reduces workload for the operator of a spot scanner.
There is a need, therefore, for a method of providing calibration data for a printer that reduces operator workload while providing accurate results.